In the fabrication of electronic devices it is frequently necessary to provide a patterned layer of metallization, generally aluminum, which functions, for example, as interconnections among devices on a substrate. One method of forming such a layer of metallization is to coat the entire suface of the substrate and then wet or dry etch the coating leaving only the desired fine line pattern. This method suffers certain disadvantages, particularly with regard to the patterning of a layer of aluminum.
Utilizing conventional lithographic techniques, a layer of resist material is deposited over the metallization layer, irradiated and developed, thus exposing the portion of the underlying metal layer to be etched away. The pattern irradiation of a resist layer with light, e.g. ultraviolet light, can suffer from a loss of resolution when the metallization is a reflective material such as aluminum. The problem of resolution loss due to reflection can be minimized by irradiating the resist layer with electron beam. Electron beam irradiation of most of the resist layer surface, however, is economically unattractive.
In either instance, dry etching of the layer of metallization is preferred over wet chemical etching due to superior resolution. However, dry etching of most of the substrate surface may be undesirable due to the possibility of damage to sensitive devices in the substrate surface. An alternative for the deposition of a patterned layer of metallization is the use of lift-off techniques.
Lift-off techniques require a multilayer structure comprising a lift-off layer and at least one overlying layer. Upon irradiation of the overlying layer or layers and development of the multilayer structure, the lift-off layer must undercut the overlying structure, ideally to a predictable degree. The undercut profile of the vias formed in the structure has two functions. First, a layer of metallization deposited over the structure will break in the vias and deposit in a pattern on the substrate which corresponds to that formed in the top layer of the lift-off structure. Second, the undercut profile enables a solvent for the lift off layer to penetrate to the substrate surface and lift off the entire structure, leaving only the patterned layer of metallization deposited in the vias. Both of these functions are essential for an effective lift-off technique.
There are, however, problems inherent in the use of lift-off techniques. For example, it is difficult to obtain structures with the necessary profile for effective lift-off with the degree of reproducibility necessary for commercial use. It has also been found that the solubility of certain materials heretofore utilized as the lift-off layer in conventional structures is significantly reduced by the heat treatments which may be required in connection with metallization deposition thereby making removal of the structure very difficult.
The presence of topography on the substrate is another factor to be contended with in developing an effective lift-off structure. The bottom or lift-off layer in such a structure tends to be thicker at the base and sides of a raised feature on the substrate and be thinner at the edges and on top. The uneven thickness of the lift-off layer over topography makes it very difficult to consistently obtain the necessary via profile for effective lift-off.
The improved process of this invention utilizes a structure which possesses superior lift-off capabilities, particularly wherein deposition of a desired material is to be on a substrate having topography.